After the American capture of the Marshall Islands, and the devastating air attacks against the Japanese fortress island of Truk Atoll in the Carolines in January 1944, the Japanese military leaders reevaluated their situation. All indications pointed to an American drive toward the Mariana Islands and the Carolines. To counter such an offensive, the Imperial Japanese Army and the Imperial Japanese Navy (I.J.N.) established an inner line of defenses extending generally northward from the Carolines to the Marianas, and thence to Japan via the Volcano Islands, and westward from the Marianas via the Carolines and the Palau Islands tothe Philippines.
The commander of the Japanese garrison on Chichi Jima was placed nominally in command of Army and Navy units in the Volcano Islands. After the American conquest of the Marianas, daily bomber raids from the Marianas hit the mainland as part of Operation Scavenger. Iwo Jima served as an early warning station that radioed reports of incoming bombers back to mainland Japan. This allowed Japanese air defenses to prepare for the arrival of American bombers.
After the U.S. seized bases in the Marshall Islands in the battles of Kwajalein and Eniwetok in February 1944, Japanese Army and Navy reinforcements were sent to Iwo Jima: 500 men from the naval base atYokosuka and 500 from Chichi Jima reached Iwo Jima during March and April 1944. At the same time, with reinforcements arriving from Chichi Jima and the home islands, the Army garrison on Iwo Jima reached a strength of more than 5,000 men. The loss of the Marianas during the summer of 1944 greatly increased the importance of the Volcano Islands for the Japanese, who were aware that the loss of these islands would facilitate American air raids against the Home Islands, disrupting war manufacturing and severely damaging civilian morale. Final Japanese plans for the defense of the Volcano Islands were overshadowed by the fact that the Imperial Japanese Navy had already lost almost all of its power, and it could not prevent American landings. Moreover, aircraft losses throughout 1944 had been so heavy that, even if war production were not affected by American air attacks, combined Japanese air strength was not expected to increase to 3,000 warplanes until March or April 1945. Even then, these planes could not be used from bases in the Home Islands against Iwo Jima because their range was not more than 900 km (560 mi). Besides this, all available warplanes had to be hoarded to defend Taiwan and the Japanese Home Islands from any attack. Adding to their woes, there was a serious shortage of properly trained and experienced pilots and other aircrew to man the warplanes Japan had—because such large numbers of pilots and crewmen had perished fighting over the Solomon Islands and during the Battle of the Philippine Sea in mid-1944.
In a postwar study, Japanese staff officers described the strategy that was used in the defense of Iwo Jima in the following terms:
In the light of the above situation, seeing that it was impossible to conduct our air, sea, and ground/ operations on Iwo Island [Jima] toward ultimate victory, it was decided that to gain time necessary for the preparation of the Homeland defense, our forces should rely solely upon the established defensive equipment in that area, checking the enemy by delaying tactics. Even the suicidal attacks by small groups of our Army and Navy airplanes, the surprise attacks by our submarines, and the actions of parachute units, although effective, could be regarded only as a strategical ruse on our part. It was a most depressing thought that we had no available means left for the exploitation of the strategical opportunities which might from time to time occur in the course of these operations.
— Japanese Monograph No. 48
At the end of the Battle of Leyte in the Philippines, the Allies were left with a two-month lull in their offensive operations before the planned invasion of Okinawa. Iwo Jima was strategically important: it provided anair base for Japanese fighter planes to intercept long-range B-29 Superfortress bombers, and it provided a haven for Japanese naval units in dire need of any support available. In addition, it was used by the Japanese to stage air attacks on the Mariana Islands from November 1944 through January 1945. The capture of Iwo Jima would eliminate these problems and provide a staging area for Operation Downfall – the eventual invasion of the Japanese Home Islands. The distance of B-29 raids could (hypothetically) be cut in half, and a base would be available for P-51 Mustang fighters to escort and protect the bombers.
American intelligence sources were confident that Iwo Jima would fall in one week. In light of the optimistic intelligence reports, the decision was made to invade Iwo Jima and the operation was given the code name Operation Detachment. American forces were unaware that the Japanese were preparing a complex and deep defense, radically departing from their usual strategy of a beach defense. So successful was the Japanese preparation that it was discovered after the battle that the hundreds of tons of Allied bombs and thousands of rounds of heavy naval gunfire had left the Japanese defenders almost undamaged and ready to inflict losses on the U.S. Marines